1. Field of the Invention
This invention relates to improvement in a system for driving meters such as a speedometer or a tachometer in response to an output from a speed or rotation sensor, and so on.
2. Description of the Related Art
A plurality of meters are used in a vehicle. They include a speedometer, a tachometer, a water temperature gauge, an oil pressure gauge, a boost gauge, and so on. These meters are operated by a driving system.
FIG. 8 of the accompanying drawings shows the configuration of an example of a conventional meter driving system. The meter driving system operates a cross-coil meter 10. The cross-coil meter 10 includes two coils which are crossed as shown in FIG. 9. A meter pointer is moved in response to a current supplied to these coils.
The two excitation coils L.sub.S and L.sub.C generate magnetic fields having desired angles and orientations when a pulse current having a pulse width corresponding to input data such as speed is supplied to the coils. A rotatable permanent magnet M receives torque generated by the magnetic fields of the coils L.sub.S and L.sub.C.
The pointer connected to the magnet M moves by angle .theta. from a reference position so as to indicate a value (a speed in FIG. 10), according to the input, on a meter panel.
The meter driving system of FIG. 8 comprises a period counter 12, a CPU 14, a reference clock generator 16, a ROM 18, a RAM 20, a digital to analog converter 24, a driver 26, an address bus 28 and a data bus 30.
The period counter 12 receives a pulse output from a sensor, and measures a period of the pulse. It is assumed here that the pulse period corresponds to a detected result (e.g. speed). In this case, the period counter 12 measures the interval (period) of the leading edge or the trailing edge of the pulse based on the number of the reference clocks, which are periodically generated by the reference clock generator 16. The period of the sensor output measured by the period counter 12 is transmitted to the data bus 30 as data, which is then forwarded to the CPU 14. In this case, the period is measured instead of frequency so as to be compatible with a low frequency output from the sensor.
The CPU 14 processes the output from the period counter 12 according to a program stored in the ROM 18. The RAM 20 is used as a work area for the CPU 14. The calculated result of the CPU 14 is sent to the data bus 30 so as to be latched in the register 22.
The register 22 latches the output from the CPU 14, supplying the output to the digital to analog converter 24 serving as a PWM modulator. The CPU 14's output which is subject to the digital to analog conversion is supplied to the driver 26. The driver 26 operates the cross-coil meter 10 accordingly. In other words, the output current from the driver 26 flows through the coils L.sub.S and L.sub.C.
With the foregoing circuit, the periods of sensor outputs are discretely measured, so that indication of the measured values are sometimes given with delay. For example, when the frequency of output pulses decreases as shown by the broken line in FIG. 11, the indication of the measured result will vary stepwise on the cross-coil meter 10 as shown by the solid line. Therefore, the indication of the cross-coil meter 10 is delayed by the amount which corresponds to the difference between the value shown by the broken line and that shown by the solid line. When no pulse output arrives from the sensor, the period counter 12 cannot count the pulse, so that the indication remains the same on the cross-coil meter 10.